Since 1972, Hounsfield invented the first CT machine, CT technology has brought a revolutionary impact to medical diagnosis and nondestructive testing in industry and CT has become one important means of detection in the medical, biotechnology, aerospace and defense sectors and the like industries. As technology advances, scan modes and imaging methods for CT are continually improving, 3D cone-beam CT has become the mainstream of research and application. X-ray cone-beam CT has been widely used in clinical, safety inspection, NDT (Non-Destructive Test) and other fields. Especially in clinical diagnosis, helical CT has become an indispensable means of inspection.
In 1989, spiral CT was put into clinical practices. Due to the advantages of spiral CT, it has gradually replaced the previous plane CT. Compared with plane CT, spiral CT has the following advantages: spiral CT can uninterruptedly collect projection data and get 3D volume data of an object through a specifically designed reconstruction algorithm so as to significantly shorten the time for CT scanning, improve the Z-axis resolution of a reconstructed image and reduce motion artifacts. In 1991, the company, Elscint, firstly introduced the double slice CT on basis of the single slice CT, hence opening a prelude to the rapid development of Multi-slice spiral CT (i.e., Multi-slice CT, MSCT).
MSCT differs from the single slice spiral CT in that the detectors of the single slice CT are arranged in a single row and can collect only one slice of fan-beam projection data at a time, while MSCT has a multi-row arrangement of detectors and thus can simultaneously collect several slices of cone-beam projection data. Thus, compared with the single slice spiral CT, MSCT has been greatly enhanced in performance, specifically with the coverage of X-ray beam being greatly increased, the utilization of X-ray being effectively improved and the scanning time being shortened. This leads to 3D reconstructed images with higher quality. In 1998, companies, such as GE, Siemens, Toshiba and Philips, introduced a 4-slice spiral CT, hence opening a prelude to fierce competitions among these major medical device companies. In 2001, GE pioneered an 8-slice spiral CT. In 2002, GE, Siemens, Toshiba and Philips respectively launched a 16-slice spiral CT. In 2005, Toshiba introduced the 256-slice spiral CT. In 2007, in the 93rd conference of the North American radiology in Chicago, US, Toshiba announced the launch of its new 320-slice spiral CT products. The Siemens launched in 2005 the first dual-energy spiral CT. The scanning speed of the latest MSCT has been up to 4 turns per second, and has been widely used in 3D imaging for human body, angiography, cardiac imaging, perfusion imaging and the like. On basis of MSCT technology, new technologies like computer-aided surgery, virtual endoscopy and radiotherapy have been emerged and developed.
Although CT technology has achieved great successes in industry, security, medical and other fields, due to complexity and diversity of engineering application conditions, higher requirements poses a need for further development of CT technology. During CT scanning, motions of the scanned object frequently occur, especially in CT imaging of living bodies. For example, during medical CT scanning, due to the inherent motions of organs, there are often motion artifacts in CT images, leading to image blurring, ghosting, etc., which may affect the accuracy of physician's diagnosis of diseases. There are a variety of physiological motions for human organs in the normal metabolism course, including: heart beating, lungs breathing exercises, pulses, gastrointestinal motility and the like. Among the motions of these organs, except for breathing may be controlled by holding one's breath, it is difficult to temporarily stop most of the other motions. In addition, for some special sections of population that are unable to control their own body movements, such as Parkinson's patients, infants, mental patients without self-control and the like, noticeable limb movements may occur during CT scanning. These movements inevitably lead to motion artifacts in MSCT image, poor picture quality, and even a failed scanning. Therefore, studies on reduction and elimination of motion artifacts are of great significance for CT imaging.
In the case of holding one's breath, a healthy human body has an average movement displacement of about 0.35 mm in 10 seconds, which may not lead to serious motion artifacts in the image since the current MSCT image has a resolution of only 0.4 mm. However, the organs in a human's torso are influenced by heart beat, gastrointestinal peristalsis and the like. These organs have more obvious movements than that of the head. Especially for patients, since their ability to control their bodies have been weakened, the range of movement may be more than 1 mm or even more, thereby leading to severe motion artifacts while affecting quality of CT images.
A fundamental way to reduce motion artifacts is to improve imaging speed so as to reduce the impact caused by organ motions in the scanning process. However, the existing MSCT enables X-ray machines and detectors to rapidly rotate around the body through the slip ring technology, so as to carry out the CT scanning. Currently, the highest scanning speed is 0.25 second/circle. Due to a variety of restrictions such as centrifugal force, material strength and the like, this speed has already achieve the uppermost limit in industry applications, and thus it is difficult to enable a breakthrough in next few years. In addition to MSCT, speeds for other medical CT and industrial CT scanning are far lower than 0.25 seconds/circle. Thus, it is impossible to eliminate motion artifacts by simply increasing the scanning speed.
Another existing technology for suppressing motion artifacts is Gating technology, which has been applied in cardiac imaging. The Gating technology groups MSCT scanning data in accordance with the heart phases, performs image reconstruction for the groups, respectively, and then gets a final CT image by using image fusion technology. The problems with Gating technology are that some X-ray projection data are not effectively used, leading to an increased radiation dose used in cardiac imaging. Medical radiation has become the topmost man-made ionizing radiation, and reduction of X-ray dose in CT inspection is a major issue related to health of the public and future generations.